This invention pertains generally to rotation rate sensors and, more particularly, to a differential charge amplifier with a built-in test circuit for use in a rotation rate sensor.
Rate sensors with piezoelectric structures such as tuning forks rely on the Coriolis effect to sense rotation. The drive side of the tuning fork is driven in an oscillator circuit, with an automatic gain control (AGC) circuit keeping the current to the drive crystal constant. When the tuning fork is rotated, the pick-up tines develop an out-of-plane mode of vibration due to the Coriolis force. This vibration is detected piezoelectrically, and the resulting charge signal is directly proportional to the angular rate of rotation. That signal is converted from a charge signal to a voltage signal in a device known as a charge amplifier.
A commonly used charge amplifier circuit is shown in FIG. 1. This is a single-ended circuit in which the pick-up high electrode on the tuning fork is connected to the inverting input of the charge amplifier QA1, and the pick-up low electrode is connected to virtual ground Vg. A feedback resistor Rf and a feedback capacitor Cf are connected between the output and the inverting input of the amplifier. The resistive element provides a DC feedback path, and the capacitive element provides AC feedback. The capacitive element also provides the transfer function for the charge signal which is proportional to the angular rate at the drive fork frequency:
Vout(t)=xe2x88x92qm(t)/Cf.
The non-inverting input of charge amplifier QA1 is also connected to virtual ground, and with a unipolar power supply, virtual ground is set to be one-half of the supply voltage in order to maximize the dynamic range of the amplifier. With a bipolar power supply, the pick-up low electrode and the non-inverting input of the charge amplifier are typically connected to a ground reference instead of the virtual ground.
FIG. 2 illustrates a single-ended charge amplifier with a built-in test circuit, as disclosed in U.S. Pat. No. 5,426,970, the disclosure of which is incorporated herein by reference. In this system, a continuous built-in test signal CBIT is coupled to the pick-up output of the tuning fork through the pick-up low electrode. This signal is an attenuated version of the drive signal for the tuning fork and when superimposed on the output of the fork, it acts as a large AC bias signal. The CBIT bias passes through all elements of the signal path in the rate sensor until it is subtracted out either in software or in hardware.
By this process, the tuning fork and all of the gain stages in the forward rate channel are verified to be functional. If any of these elements should fail, the CBIT bias at the output will not be equal and opposite to the cancellation signal, and this shift in output is interpreted as an indication of the failure.
Being unbalanced circuits, the charge amplifiers of FIGS. 1 and 2 are more susceptible to common-mode noise than a balanced circuit would be. In addition, noise gain is a function of stray capacitance on the inverting input of the amplifier, and the DC offset of the amplifier usually needs to be blocked in subsequent stages in order to preserve dynamic range and linearity.
FIG. 3 illustrates a differential charge amplifier which provides a balanced circuit for differential measurement of the pick-up fork signal. In this circuit, the two pick-up electrodes are connected to the inputs of differential amplifier QA1, resistor Rd1 and capacitor Cd1 are connected between the output and the inverting input, and resistor Rd2 and capacitor Cd2 are connected between the non-inverting input and virtual ground, with Rd2 being equal to Rd1, and capacitor Cd2 being equal to Cd1. In this circuit, charge or current once again flows through the feedback elements, converting the output to a voltage-mode signal, but with two signal paths of equal impedance, one for each output of the pick-up fork.
The output of the differential amplifier can be either differential or single-ended. Although the charge amplifier of FIG. 3 is shown as being referenced to virtual ground for a unipolar power supply, a power ground reference is typically used with a bipolar supply.
The differential charge amplifier has several advantages over a single-ended circuit. It provides a 6 dB increase in signal-to-noise ratio (SNR) due to the gain of 2 in the differential circuit. The balanced structure provides increased common-mode rejection, which attenuates common-mode noise and further increases SNR. DC offset is also greatly attenuated because the circuit has a large common-mode rejection at DC.
The differential charge amplifier of FIG. 3 also has certain limitations and disadvantages. The common-mode rejection of the circuit is dominated by the matching of the RC passive components as opposed to the common mode rejection of the operational amplifier. Moreover, the circuit cannot be configured for continuous built-in testing. The circuit is balanced for differential detection of a charge such as a rate signal, but any voltage signal appearing on either electrode of the pick-up fork will automatically be present on the other electrode due to the virtual input of the operational amplifier.
It is in general an object of the invention to provide a new and improved charge amplifier.
Another object of the invention is to provide a charge amplifier of the above character which overcomes the limitations and disadvantages of the prior art.
These and other objects are achieved in accordance with the invention by providing a differential charge amplifier for processing charge signals from a rotation rate sensor, and means for applying a test signal to the differential charge amplifier so that during normal operation the output of the amplifier corresponds to the test signal as well as to the charge signals.